4 Reales - Philip IV

The Potosí mint's cob coinage — macuquinas — from this precise window sits at the center of one of the most consequential numismatic scandals in colonial history. In 1649, assayer Francisco de la Casa was found to have been systematically debasing silver intended for royal coinage at Potosí, shaving fineness well below the mandated standard. The fraud had run for years. Crown investigations led to executions, the destruction of existing dies, and a complete administrative reorganization of the mint by 1652 — which marks the hard opening of this type's date range.

Coins struck in the years immediately following that purge carry assayer marks reflecting the new regime of oversight. The KM#R18 designation covers the rare assayer variants within the series.